MEMS resonators and method for manufacturing MEMS resonators

ABSTRACT

A first type of MEMS resonator adapted to be fabricated on a SOI wafer is provided. A second type of MEMS resonator that is fabricated using deep trench etching and occupies a small area of a semiconductor chip is taught. Overtone versions of the resonators that provide for differential input and output signal coupling are described. In particular resonators suited for differential coupling that are physically symmetric as judged from center points, and support anti-symmetric vibration modes are provided. Such resonators are robust against signal noise caused by jarring. The MEMS resonators taught by the present invention are suitable for replacing crystal oscillators, and allowing oscillators to be integrated on a semiconductor chip. An oscillator using the MEMS resonator is also provided.

FIELD OF THE INVENTION

The present invention relates to Microelectromechanical Systems (MEMS).More particularly, the present invention pertains to frequency selectiveMEMS devices, and methods for manufacturing MEMS devices.

BACKGROUND OF THE INVENTION

Currently, there is an interest in increasing the degree of integrationof electronics. Integration has proceeded steadily over the last fewdecades and achieved remarkable reduction in the physical size occupiedby electronic circuits. Semiconductor lithography, has enabled circuitswith millions of transistors to be constructed on a single silicon die.Nonetheless, certain components are difficult to integrate.

For example, inductors are difficult to integrate Although certainspiral shaped designs for integrated circuits have been proposed, owingto their inherent resistive losses, these spiral inductors are illsuited for producing high Q resonators which are needed to generatestable frequency signal sources.

One important component that is used to generate stable frequencies in avariety of electronic apparatus including sequential logic (e.g.,microprocessors) and wireless communication transceivers is the quartzcrystal resonator. The quartz crystal resonator in its usual form is abulky discrete component.

Microelectromechanical System (MEMS) based resonators have been proposedas an alternatives to quartz resonators for use as frequency selectivecomponents for use at RF frequencies. One type of MEMS resonator thathas been proposed comprises a suspended beam of semiconductor materialthat is shaped and sized to resonate at a selected frequency chosen inview of a desired electrical frequency response. The MEMS resonatorserves as a frequency selective component in a circuit. According to onedesign the MEMS resonator is driven by a drive electrode that extendsbelow the suspended beam. Electric force interaction between thesuspended beam and the drive electrode induces the suspended beam tovibrate.

Although a MEMS resonator occupies very little space compared to anexternal discrete component it does take up substantial space comparedto electrical components found on integrated circuits. A single MEMSresonator can take up space on a semiconductor die that could have beenused for tens of transistors. In some applications it would beadvantageous to be able to reduce the die area occupied by a MEMSresonator.

Another drawback of suspended beam type MEMS resonators is that they aresusceptible to shock and vibration. External shock and vibration willcause spurious electrical signals to be generated by beam type MEMSresonators.

During the past decade there has been an increased interest in thesemiconductor industry in use of Silicon On Insulator (SOI) wafers. SOIwafers include a silicon substrate, a silicon di-oxide layer on thesilicon substrate, and a single crystal silicon layer on the silicondi-oxide layer. SOI wafers afford a number of advantages in terms of theelectrical properties of circuits built using them, including reducedvoltage requirements, and power consumption for a given clock speed.

It would be advantageous to have a MEMS resonator design that isespecially suited for implementation on a SOI wafer.

The electrical impedance of a beam type MEMS resonator is determined byits geometry. It would also be advantageous for some applications, to beable to provide a MEMS resonator having reduced impedance.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow chart of a process for manufacturing a MEMS resonatoron a SOI wafer according to a preferred embodiment of the invention.

FIG. 2 is a sectional elevation view of a SOI wafer used in the processshown in FIG. 1.

FIG. 3 is a sectional elevation view of the SOI wafer shown in FIG. 2during a first resist exposure operation.

FIG. 4 is a sectional elevation view of the SOI wafer shown in FIG. 3during a doping operation.

FIG. 5 is a plan view of the SOI wafer shown in FIG. 4 after a dopingoperation.

FIG. 6 is a sectional elevation view of the SOI wafer shown in FIG. 5during a second resist exposure operation.

FIG. 7 is a sectional elevation view of the SOI wafer shown in FIG. 6after a resist development operation.

FIG. 8 is a sectional elevation view of the SOI wafer shown in FIG. 7after a silicon top layer etching operation.

FIG. 9 is a plan view of the SOI wafer shown in FIG. 7 after the silicontop layer etching operation.

FIG. 10 is a sectional elevation view of the SOI wafer shown in FIG. 9during a third resist exposure operation.

FIG. 11 is a sectional elevation view of the SOI wafer shown in FIG. 10after a resist development operation.

FIG. 12 is a sectional elevation view of the SOI wafer shown in FIG. 11after an oxide etch operation.

FIG. 13 is a broken out perspective view of a wafer showing the SOI MEMSresonator shown in FIG. 12.

FIG. 14 is a broken out perspective view of a wafer showing a second SOIMEMS resonator according to an embodiment of the invention.

FIG. 15 is a broken out perspective view of a wafer showing a third SOIMEMS resonator according to an embodiment of the invention.

FIG. 16 is a broken out perspective view of a wafer showing a fourth SOIMEMS resonator according to an embodiment of the invention.

FIG. 17 is a flow chart of a first process of making a SOI wafer.

FIG. 18 is a depiction of a silicon wafer used in making a SOI wafer.

FIG. 19 is a sectional elevation view of the wafer shown in FIG. 18after an oxide growth step.

FIG. 20 is a sectional elevation view of the wafer shown in FIG. 19after a hydrogen implantation step.

FIG. 21 is a sectional elevation view of the wafer shown in FIG. 20bonded to a second wafer of the type shown in FIG. 18.

FIG. 22 is a SOI wafer obtained by cleaving the wafer shown in FIG. 21.

FIG. 23 is a flow chart of a second process of making a SOI wafer.

FIG. 24 is a sectional elevation view of the SOI wafer made by theprocess shown in FIG. 23.

FIG. 25 is a flow chart of a third process of making a SOI wafer.

FIG. 26 depicts sectional elevation views of two wafers used to make theSOI wafer according to the process shown in FIG. 25.

FIG. 27 is a sectional elevation view of a SOI wafer produced by theprocess shown in FIG. 25.

FIG. 28 is a sectional elevation view of a wafer bearing a first resistthat is being exposed to patterning radiation in a process for making aMEMS resonator.

FIG. 29 is a sectional elevation view of the wafer shown in FIG. 28during a doping operation.

FIG. 30 is a plan view of the wafer shown in FIG. 29 showing dopedareas.

FIG. 31 is a sectional elevation view of the wafer shown in FIG. 29bearing a second resist that is being exposed to patterning radiation.

FIG. 32 is a sectional elevation view of the wafer shown in FIG. 31after development of the second resist.

FIG. 33 is a sectional elevation view of the wafer shown in FIG. 32after etching using the second resist.

FIG. 34 is a plan view of a first vertically oriented resonant memberMEMS resonator device.

FIG. 35 is a flow chart of a process of making a MEMS resonatoraccording to an embodiment of the invention.

FIG. 36 is a fragmentary plan view of a MEMS resonator that hasvibrating plate oriented perpendicular to a semiconductor chip surface.

FIG. 37 is a sectional elevation view of the MEMS resonator shown inFIG. 36.

FIG. 38 is a fragmentary plan view of a MEMS resonator that has acorrugated trench wall.

FIG. 39 is a fragmentary plan view of a MEMS resonator that includes avibrating plate with two clamped edges.

FIG. 40 is a fragmentary plan view of a MEMS resonator that includes avibrating plate with three clamped edges.

FIG. 41 is a schematic of an oscillator using the MEMS resonator shownin FIG. 16.

FIG. 42 is a schematic of an oscillator using the MEMS resonator shownin FIG. 40.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While this invention is susceptible of embodiment in many differentforms, there are shown in the drawings and will herein be described indetail specific embodiments, with the understanding that the presentdisclosure is to be considered as an example of the principles of theinvention and not intended to limit the invention to the specificembodiments shown and described. Further, the terms and words usedherein are not to be considered limiting, but rather merely descriptive.In the description below, like reference numbers are used to describethe same, similar, or corresponding parts in the several views of thedrawings.

According to certain preferred embodiments of the present invention aMEMS resonator design and method of manufacture are provided that areespecially suited to implementation on a SOI wafer. The design requiresonly the silicon layer normally present on the surface of a SOI waferfor construction of the basic resonator device-it does not requiredoping of the underlying substrate to provide electrically active areas,or deposition of additional semiconductor layers. Thus in combinationwith the inherent advantages of SOI which are well suited to making lowpower consumption devices, the SOI MEMS resonator taught by the presentinvention opens up the possibility of making highly integrated low powerelectronic devices at low cost. The combination of low cost and lowpower will enable the further proliferation of electronic devices (e.g.,low cost wireless communication devices).

According to other embodiments of the invention a MEMS resonator designthat requires very little area on a silicon die is provided. By reducingthe area required for a die for a given device, the number of die thatcan be fit on a wafer can be increased, and the cost per device can bedecreased proportionately.

FIG. 1 is a flow chart of a process 100 for manufacturing a MEMSresonator on a SOI wafer according to a preferred embodiment of theinvention.

In step 102 a SOI wafer is obtained. SOI wafers can be produced using anumber of manufacturing processes including the UNIBOND™ process, theSeparation by Implantation with Oxygen (SIMOX), and the Bond and EtchBack Silicon on Insulator (BESOI) process. These processes are describedbelow in further detail. SOI wafers are available commercially. Incarrying out the invention SOI wafers would likely be obtainedcommercially and not produced in-house. UNIBOND™ SOI wafers areavailable commercially from SOITEC USA of Peabody, Mass. SIMOX SOIwafers are available from IBIS corporation of Danvers, Mass.

FIG. 2 is a sectional elevation view of a SOI wafer 200 used in theprocess shown in FIG. 1. (Note that due to the differences in scalebetween wafers and devices fabricated thereon, the sectional elevationviews shown in the figures are not draw to scale.) The SOI wafer 200comprises a silicon base layer 202, an silicon di-oxide layer 204 bornon the silicon base layer 202, and a single crystal silicon (device)layer 206 born on the oxide layer 204. The single crystal silicon layers206 on SOI wafers 200 have a low residual stress. This property is usedto advantage in the present invention in which resonator beams can beetched out of the silicon layer 206 without ensuing deformation due toresidual stress. Due to the lack of residual stress in the silicon layer206 lengthy annealing prior to etching is not be required. However,annealing may be performed as part of the process of manufacturing theSOI wafer 200.

Referring again to FIG. 1 in step 104 a resist 302 (FIG. 3) is appliedto the SOI wafer 200. In a commercial implementation the resist wouldlikely be a photoresist that is suited to UV or X-ray exposure. Forprototyping an e-beam resist and e-beam resist patterning is preferred.If needed the resist can be softbaked after it has been applied toevaporate a portion of a solvent component of the resist.

In step 106 the first resist 302 (FIG. 3) is exposed using a first mask304 (FIG. 3). The first mask 304 (FIG. 3) determines a pattern of dopingof the single crystal silicon layer 206.

FIG. 3 is a sectional elevation view of the SOI wafer 200 shown in FIG.2 during a first resist exposure operation. As shown in FIG. 3, thefirst resist 302 has been applied to the wafer 200. The wafer 200 can besupported on the stage of a stepper (not shown) proximate to a firstexposure mask 304. Radiant or corpuscular energy (e.g., ultraviolet,X-ray or free electrons) 308 is used to image the mask 304 onto theresist 302. The mask 304 can for example be a phase shift mask in thecase that deep ultraviolet is used.

Referring once again to FIG. 1 in step 108 the first resist 302 (FIG. 3)is developed. Optionally the resist can be hard baked after developmentin preparation for further processing. In step 110 the silicon layer 206is doped to define conductive pathways onto a resonant member. Note thatat this point in the processing the outline of the resonant member hasyet to be etched.

FIG. 4 is a sectional elevation view of the SOI wafer shown in FIG. 3during a doping operation. In FIG. 4 the resist 302 is shown afterdevelopment in a patterned state. A flux of dopant species (e.g., atomsor ions) 402 is shown above the wafer. Preferably doping is accomplishedusing an ion implanter, as that is the tool of choice in modernsemiconductor fabrication facilities. Alternatively vapor phase dopingin a diffusion furnace can be used.

FIG. 5 is a plan view of the SOI wafer 200 shown in FIG. 4 after thedoping operation. The location of the section plane of FIG. 4 isindicated in FIG. 5. As seen in FIG. 5, after the doping step 110, theSOI wafer 200 includes a first doped region 502, and a second dopedregion 504 separated by an non-doped insulating (isolating) region 506.The insulating region can have a low conductivity due to a low dopantconcentration. The insulating region can have a sufficient dopant tomake its conductivity significant, yet still serve as an isolatingregion if its dopant is opposite in type (e.g. P as opposed to N) tothat used in the first 502 and second 504 doped regions. In the lattercase isolation is provided by the presence of at least one reversedbiased PN junction between the first 502 and second doped regions, forany voltage difference between the two doped regions 502, 504. The firstdoped region 502 includes a first sub region 502A that in the completedMEMS resonator will be located on a resonating member, an elongated subregion 502B that in the completed MEMS resonator will lie along asupport beam. At the end of the elongated sub region is a pad shapeddoped sub region 502C that in the completed MEMS resonator will belocated on a perimeter ring that will support the support beam.Similarly the second doped region includes corresponding sub-regions504A, 504B, and 504C.

Referring to FIG. 2 in step 112 the first resist 302 is stripped fromthe SOI wafer 200, and in step 114 a second resist 602 (FIG. 6) isapplied to the SOI wafer 200. In step 116 the second resist 602 isimagewise exposed to corpuscular or radiant energy using a second mask604 (FIG. 6). The second resist 602 defines a pattern for etching thesingle crystal silicon 206 layer.

In step 118, the second resist layer 602 (FIG. 6) is developed. Thedeveloped second resist layer is shown in FIG. 7. In step 120 the singlecrystal silicon layer 206 is patternwise etched to define a beam shapedmember 802 (FIG. 8) capable of resonating in a vibrational mode and oneor more supports attached to the member. FIG. 8 is a sectional elevationview showing the resonating member 802, and a perimeter ring 804 thatalong with a plurality of support beams (not visible in this view)support the resonating member 802.

FIG. 9 is a plan view of the SOI wafer 200 shown in FIG. 7 after thesilicon top layer etching operation. As shown in FIG. 9, the first dopedsub region 502A and the second doped sub region 504A are located on theresonating member 802, separated by the isolation region 506. Theresonating member 802 is seen to be in the form of an elongated beam.The resonating member 802 is attached to the peripheral ring 804 by twosupport beams 902, 904 which extend perpendicularly from opposite sidesof the resonating beam 802 at its longitudinal center. Conducting subregions 502A, 504A are on the two support beams 902, 904 respectively.The section plane of FIG. 8 is indicated in FIG. 9.

Referring once again to FIG. 1, in step 122 the second resist 602 (FIG.6) is removed, and in step 124 a third resist 1002 (FIG. 10) is applied.The third resist 1002 (FIG. 10) is used to define an area for etchingthe oxide layer 204. In step 126 the third resist 1002 (FIG. 10) isexposed to corpuscular or radiant energy 308 (FIG. 3) using a third mask1004 (FIG. 10). In step 128 the third resist 1002 (FIG. 10) isdeveloped. The third resist 1002 (FIG. 10) is shown after development inFIG. 11. In step 130 the oxide 204 under the resonant member 802, (infact all of the oxide within the perimeter ring 804) is etched in orderto free the resonant member for movement. A Buffered Oxide Etch (BOE)solution is suitable for etching the oxide 214. FIG. 12 is a sectionalelevation view of the SOI wafer shown in FIG. 11 after an oxide etchoperation. The support beams 902, 904 (FIG. 9) that connect the resonantmember 802 to the peripheral ring 804 are not visible in this sectionalview (taken along the same lines indicated for FIG. 8 in FIG. 9).

FIG. 13 is a broken out perspective view of the wafer 200 showing theSOI MEMS resonator 1300 fabricated by process 100. The resonant member802 has a first end 802A, second end 802B, a first peripheral edge 802Cextending from the first end 802A to the second end 802B, and a secondperipheral edge 802D extending from the first end 802A to the second end802B. The first 902 and second 904 support beams attached atlongitudinal centers of the first 802C and second 802D peripheral edgesof the resonant member 802. The first doped region 502 including subregions 502A, 502B and 502C, and the second doped region 504 includingsub regions 504A, 504B and 504C are shown as cross hatched areas. (Dopedregions are shown as cross hatched areas in FIGS. 13-16,30,34,36,38-40.Cross hatched areas in other view may represent different regions asdescribed.) Other parts indicated by reference numeral are describedabove with reference to the foregoing figures.

FIG. 14 is a broken out perspective view of a wafer showing a second SOIMEMS resonator 1400 according to a preferred embodiment of theinvention.

The MEMS resonator 1400 comprises a peripheral ring 1402 of singlecrystal silicon 1402 born on a silicon dioxide layer 204. The silicondioxide layer 204 is borne on an underlying silicon substrate 202. Asingle crystal silicon beam shaped resonant member 1412 is centeredwithin the peripheral ring 1402. The beam shaped resonant member has afirst end 1412A, a second end 1412B, a first longitudinal edge 1412Cextending between the first end 1412A, and the second end 1412B, and asecond longitudinal edge 1412D extending between the first end 1412A andthe second end 1412B. First 1404, second 1406, third 1408, and fourth1410 support beams extend between the peripheral ring 1402 and theresonant member 1412. The support beams 1404-1410 are perpendicular tothe resonant member 1412. The first 1404 and second 1406 support beamsattach to the first longitudinal edge 1412C. The third 1408 and fourth1410 support beams attach the second longitudinal edge 1412D.

The resonant member 1412 has a size and shape chosen so that it iscapable of vibrating in a predetermined mode that has first and secondnodes equally spaced from and on opposite sides of a longitudinal centerof the beam shaped resonant member 1412. The vibrational mode is a oneperiod sinusoidal flexural mode that is symmetric about the longitudinalcenter of the beam. The first 1404 and fourth 1410 support beams attachto the resonant member 1412 at the position of the first node of thesinusoid. The second 1406 and third 1408 support beams attach to theresonant member 1412 at the position of the second node of the sinusoid.The center of the beam shaped resonant member is an anti-node of thesinusoid.

A doped region (shown as a cross hatched area) 1414 extends from theperipheral ring 1402, along the first support beam 1404 to the beamshaped resonant member 1412, along the first peripheral edge 1412Ctoward the longitudinal center of the resonant member 1412, across theresonant member 1412 to the second peripheral edge 1412D, along thesecond peripheral edge 1412D toward the juncture of the third supportbeam 1408 and the resonant member 1412, and along the third supportbeams 1408 back onto the peripheral ring 1402. Portions of the dopedregion 1414 on the peripheral ring can be used to make a connectionbetween the MEMS resonator 1400 and an external circuit (not shown inthis view) such as an oscillator circuit that uses the MEMS resonator toset a resonant frequency. The external circuit can be implemented on theSOI die used to fabricate the MEMS resonator. The external circuit canbe implemented using standard methods for integrated circuitfabrication. The connection to the external circuit can be made by anohmic contact between a metallization plug (not shown) and the dopedregion 1414 e.g. at the peripheral ring 1402.

FIG. 15 is a broken out perspective view of a wafer showing a third SOIMEMS resonator 1500 according to an embodiment of the invention. Theresonator 1500 includes a beam shaped resonant member 1516. The resonantmember 1516 is shaped and sized to vibrate in a one and one-halfwavelength sinusoidal flexural beam mode that is anti-symmetric asjudged from its longitudinal center. The beam mode includes three nodes,one of which is located at the longitudinal center of the beam, and theother two of which are equally spaced from and on opposite sides of thelongitudinal center. The beam mode includes four anti-nodes two of whichare located between the central node and each of the other two nodes,and two of which are located at first and second ends 1516C, and 1516Dof the resonant member 1516. The beam shaped resonant member 1516 has, afirst longitudinal edge 1516A extending between the first end 1516C andsecond end 1516D, and a second longitudinal edge 1516B extending betweenthe first end 1516C and the second end 1516D. A first support beam 1504is connected to the first longitudinal edge 1516A at the position of afirst node. A second support beam 1506 is connected to the firstlongitudinal edge 1516A at the position of the center node. A thirdsupport beam 1508 is connected to the first longitudinal edge 1516A atthe position of a third node. Fourth through sixth support beams1510,1512, 1514 are connected to the second longitudinal edge at thepositions of the first, center, and third nodes respectively. Thesupport beams 1504-1514 extend perpendicularly away from the beam shapedresonant member 1516 to a peripheral ring 1502. The beam shaped resonantmember 1516, the peripheral ring 1502, and the support beams 1504-1514are all made from the top silicon layer 206 of a SOI wafer. Within theperipheral ring 1502, the silicon dioxide layer 204 has been etched awayto make room for the resonant member 1516 to vibrate.

A first doped region 1518 extends from the peripheral ring 1502, downthe length of the first support beam 1504, onto the resonant member1516, along the first longitudinal edge 1516A in the direction of itslongitudinal center to a first anti-node, across the resonant member1516 at the first anti-node, to the longitudinal center along the secondlongitudinal edge 1516B, along the fourth support beam 1512 to theperipheral ring 1502. Similarly, a second doped region 1520 extends fromthe peripheral ring 1502, down the length of the second support beam1506, onto the resonant member 1516, along the first longitudinal edge1516A in the direction of the second end 1516D to a second anti-node,across the resonant member 1516 at the second anti-node, along thesecond longitudinal edge to the node at which the third 1508 and sixth1514 support beams are connected, along the sixth support beam 1514 tothe peripheral ring 1502. Thus the first doped region crosses theresonant member 1516 at first anti-node adjacent to the longitudinalcenter of the resonant member 1516, and a second doped region crossesthe resonant member at a second anti-node adjacent to the longitudinalcenter of the resonant member 1516. A non-doped isolation region 1522 islocated between the first doped region 1518 and the second doped region1520.

In as much as the resonant mode of the resonant member 1516 isanti-symmetric as judged from the center of the resonant member, the twoanti-nodes at which the first and second doped regions cross theresonant member 1516 have opposite phase (i.e. when one is deflected upthe other is deflected down and visa versa). The resonator 1500 can becaused to resonate by applying opposite polarity signals to the twodoped regions 1518 and 1520. The resonator can be used a frequencyselective circuit element in a positive feedback loop of an oscillatorby connecting one side of the circuit (e.g., from the oscillatorsamplifier output) to the first conductive region 1518 and a second sideof the circuit (e.g. the oscillators amplifiers input) to the secondconductive region 1520. Connected as describe the resonator 1500 servesa role analogous to that of a quartz crystal resonator.

FIG. 16 is a broken out perspective view of a SOI wafer showing a fourthSOI MEMS resonator 1600 according to an embodiment of the invention. Thefourth resonator 1600 includes a beam shaped resonant member 1604 thatis sized and shape to oscillate at a predetermined frequency, in a twoand one-half period sinusoid flexural beam mode that is anti-symmetricas judged from a center 1642 of member 1604. The resonant member has afirst end 1604A, second end 1604B, a first longitudinal edge 1604Cextending between the first end 1604A and the second end 1604B, and asecond longitudinal edge 1604D extending between the first end 1604A andthe second end 1604B. Five support beams extend perpendicularly from thefirst longitudinal edge 1604C at positions of nodes of the abovementioned mode. In order, from the first end 1604A to the second end1604B, the five support beams are identified by reference numerals 1606,1608, 1610, 1612, and 1614. Similarly five more support beams areattached to the second longitudinal edge 1604D at positions of thenodes. These elements in order from the second end 1604B are labeled byreference numerals 1616, 1618, 1620, 1622, and 1624. The ten supportbeams 1606-1624 terminate at a peripheral ring 1602. According to analternative embodiment of the invention one or more support beams at oneor more nodes are eliminated. The resonant device 1600 includes fourdoped regions, 1626, 1628, 1630, and 1632. Each doped region extendsfrom a support beam connected to the first longitudinal edge 1604C overthe resonant member 1604, to a support beam connected to the secondlongitudinal edge 1604D that is offset from the support beam connectedto the first longitudinal edge 1604C that shares the same doped region.Each doped region crosses over an anti-node of the resonant mode. Thusfour anti-nodes are crossed. Adjacent anti-nodes have opposite phase.Every other anti-node has the same phase. The doped regions that crossover anti-nodes that have the same phase can in some embodiments beadvantageously connected to an external circuit (e.g., oscillator) inparallel. That is all the doped regions that cross anti-nodes that haveone phase can be connected to one side of the circuit, and all the dopedregions that cross over anti-nodes with the opposite phase can beconnected to the other side of the circuit. By connecting the anti-nodesto an external circuit in parallel, a lower effective impedance for theresonator is realized. This is particularly important in circuits thatrequire lower impedance circuit elements.

Alternatively, the resonator 1600 can be attached to an external circuitas a delay line. To use the resonator 1600 as a delay line, the twodoped conductive regions 1632, 1630 near the first end 1604A can be usedas differential signal inputs, and the two doped conductive regions 1628and 1626 near the second end 1604B can be used as differential signaloutputs. Alternatively the doped conductive region 1632 closest to thefirst end 1604A can be used as a single signal input, and the dopedconductive region 1626 near the second end 1604B can be used as a singlesignal output.

Alternatively one pair of oppositely phased conductive regions e.g.,1626, 1632 can be used as differential inputs of an external circuit,and the other pair of conductive regions e.g., 1628, 1630 can be used asdifferential outputs or visa versa. In this configuration the effect ofjarring of the resonator 1600 on an output signal will be reduced. Thisis explained as follows. The resonant member is physically symmetric sothat its center of gravity is located at its center 1642. The resonantmember 1604 resonates in a mode that is anti-symmetric as judged fromits center 1642. External jarring will tend to cause the center of theresonant member to deflect up and/or down in a symmetric manner whichwill cause equal movement of the above mentioned pairs of oppositelyphased doped conductive regions. If for example a differential amplifierwith a high common mode rejection ration (CMRR) is connected to a pairof oppositely phased doped conductive regions (e.g., 1628, 1630) thatare equidistant from the center, the perturbation of the signal causedby the jarring will be rejected by the differential amplifier.

Either for use as a resonator, or as a delay line, the resonator 1600can be extended so as to resonate in a higher order mode than that shownin FIG. 16.

In each of the embodiments shown in FIGS. 13-16, the peripheral ring 804(FIGS. 8,13), 1402 (FIG. 14), 1502 (FIG. 15), 1602 (FIG. 16), theresonant member 802 (FIGS. 8,13), 1412 (FIG. 14), 1516 (FIG. 15), 1604(FIG. 16) and the support beams 902-904 (FIGS. 9,13), 1404-1410 (FIG.14), 1504-1514 (FIG. 15), 1606-1624 (FIG. 16) are unitary. That is tosay that they are all etched from the top silicon layer 206 (FIG. 2) ofa SOI wafer 200 (FIG. 2).

FIG. 17 is a flow chart of a process of making the SOI wafer 200 (FIG.2) obtained in step 102 (FIG. 1). In step 1702 a first silicon wafer isobtained. FIG. 18 is a depiction of a silicon wafer 1800 used in makinga SOI wafer. The wafer includes a disk of silicon 1802. In step 1704 anoxide layer 1902 (FIG. 19) is formed on the silicon disk 1802. The oxidelayer 1902 (FIG. 19) is preferably thermally grown. FIG. 19 is asectional elevation view of the oxidized wafer 1800. The wafer 1800 hasa top layer of oxide 1902. The oxide layer 1902 may in fact cover thebottom of the wafer 1800 but a bottom layer of oxide is not critical. Instep 1706 hydrogen is implanted into the oxidized wafer at apredetermined average penetration depth below the oxidized layer 1902.FIG. 20 is sectional elevation view of the wafer 1800 after the hydrogenimplantation step. The wafer 1800 now comprises the top oxide layer1902, and upper 1800A, and lower 1800B silicon layers, separated by ahydrogen implanted silicon layer 2002. In step 1708 the implanted, andoxidized side of the wafer 1800 is placed in contact with a second wafer2102 of the kind depicted in FIG. 18, and the two wafers adhere by VanDer Waals forces to form a bonded wafer 2100. FIG. 21 is a sectionalelevation of the wafer depicted in FIG. 20 contacting a second wafer. Instep 1710 the bonded wafer is heated to about 500 C. The heating causesthe defects caused by the hydrogen implantation and/or included hydrogento coalesce thereby cleaving the wafer at about the predeterminedaverage depth of the hydrogen implant. FIG. 22 is a SOI wafer 2200obtained by cleaving the wafer shown in FIG. 21. The SOI wafer comprisesthe upper silicon layer 1800A, as an upper device layer, and the secondwafer 2102 as abase, and the oxide layer 1902 interposed between theupper silicon layer 1800A and the second wafer 2102. In step 1712 theSOI wafer is given a high temperature anneal in an inert atmosphere at1000 C. to 1300 C. for 30 minutes to 2 hours to improve bonding amongthe oxide layer and the second wafer 2102.

FIG. 23 is a flow chart of a second process 2300 of making a SOI wafer.In step 2302 a silicon wafer is obtained. In step 2304 the wafer isimplanted with oxygen to form a buried oxide layer. In step 2306 thewafer is annealed to repair damage to the crystal structure caused bythe implanting step.

FIG. 24 is a sectional elevation view of the SOI wafer 2400 made by theprocess shown in FIG. 23. The wafer 2400 comprises a silicon base 2402B,an oxide layer 2404 formed by oxygen implantation overlaying the siliconbase 2402, and a top layer of silicon 2402A overlying the oxide layer2404.

A third process 2500 for making a SOI wafer will presently be describedwith reference to FIGS. 25-27. The third process is a variant of theBESOI process mentioned above. Wafers manufactured by this process orsimilar processes are available from Canon U.S.A., Inc., of LakeSuccess, N.Y.

FIG. 25 is a flow chart of the third process 2500 of making a SOI wafer.FIG. 26 depicts sectional elevation views of two wafers produced andused in the process of making the SOI wafer shown in FIG. 25.

In step 2502 a first silicon 2600 wafer is obtained. At the start ofprocess 2500, the first silicon wafer includes a first doped singlecrystal silicon disk 2602. According to an exemplary embodiment of theinvention, the disk 2602 is P doped and has a resistivity of from about0.01 to about 0.02 ohm-cm. In step 2504 the disk 2602 is anodized toform a porous silicon layer 2604 having a thickness of from about one toabout ten microns. According to an exemplary embodiment of the inventionthe wafer is anodized in a solution of a 49% Hydrofluoric acid solutionand C2H5OH mixed in a two-to-one ratio using a current density of about7 mA/cm². In step 2506 the first wafer 2600 is oxidized in order topassivate the porous silicon layer. According to an exemplary embodimentthe wafer 2600 is oxidized in step 2508 by heating it to about 400 C.for about one hour in an oxygen atmosphere. In step 2508, the wafer isetched to remove the oxide from the surface of the porous silicon layer2604. In step 2510 a nonporous silicon layer 2606 is epitaxially grownon the surface of the porous silicon layer 2604. According to anexemplary embodiment of the invention the nonporous silicon layer 2606is grown using Chemical Vapor Deposition (CVD) in which the wafer 2600with the passivated porous silicon layer 2604 is heated to 900 C. in an80 Torr ambient of dichlorosilane and Hydrogen. In step 2512 a firstoxide layer 2608 is thermally grown on the nonporous silicon layer 2606.

In step 2514 a second wafer 2650 is obtained. Initially, the secondwafer includes a second doped silicon disk 2612. In step 2516 the secondwafer 2650 is thermally oxidized to form a second oxide layer 2610 onthe second doped silicon disk 2612. In step 2518 the oxide layer 2610 ofthe second wafer 2650 and the oxide layer 2608 of the first wafer 2600are brought into contact. In step 2520 the two contacting wafers 2600,2650 are heated in order to cause a bond to form between the two oxidelayers 2608, 2610 and produce a bonded wafer. In step 2522 the firstdisk of silicon 2602 is ground away to expose the porous silicon layer2604. In step 2524 the porous silicon layer 2604 is selectively etched,to expose the nonporous silicon layer 2606. According to an exemplaryembodiment of the invention the porous silicon layer can be selectivelyetched using a mixture of 49% Hydrofluoric acid (HF) and 30% hydrogenperoxide (H₂O₂) in a 1:5 ratio.

FIG. 27 is a sectional elevation view of a SOI wafer 2700 produced bythe process shown in FIG. 25. The SOI wafer 2700 includes the seconddoped silicon disk 2612 as a bulk layer. An oxide layer 2614 formed bybonding the oxide layer 2608 of the first wafer 2600 and the oxide layer2610 of the second wafer 2650, is born on the second doped silicon disk2612, and the nonporous silicon layer 2606 is born on the oxide layer2614.

Although three different methods for manufacturing SOI wafers have beendescribed above, the present invention is not limited to using SOIwafers made by any particular process.

Processes that are suitable for producing MEMS resonators in bulksilicon wafers will presently be described. These processes employ deeptrench etching techniques to form resonant structures that are alignedperpendicular to the surface of the wafer in which they are made. Byachieving a MEMS resonator with perpendicular orientation, it ispossible to greatly reduce the area of wafer required to accommodate aMEMS resonator. The latter economy in wafer area utilization reducesoverall manufacturing costs for a device that employs a MEMS resonator.

FIGS. 28 through 34 are a sequence of depictions of a section of a waferat which a MEMS device is being fabricated at various stages in thefabrication. These views will be referred to in the followingdescription of the fabrication process. Due to the great differences insize between a semiconductor wafer and the devices fabricated thereon,these views are not drawn to scale.

FIG. 28 is a sectional elevation view of a wafer 2806 bearing a firstresist 2804 that is being exposed to patterning radiation 2808 using afirst mask 2806 in a process for making a MEMS resonator.

FIG. 35 is a flow chart of a process 3500 of making a MEMS resonatoraccording to an embodiment of the invention. In step 3502 thesemiconductor wafer 2802 is obtained. In step 3504 the first resist 2804is applied to the semiconductor wafer 2802. In step 3506 the firstresist 2804 is imagewise exposed to radiant or corpuscular radiation2808 using the first mask 2806. In step 3506 the first resist 2804bearing wafer 2802 is soft baked to evaporate volatile solvents from theresist 2804. In step 3510 the first resist is developed. The resist 2804can optionally be hard baked after the development step 3510.

FIG. 29 is a sectional elevation view of the wafer shown in FIG. 28during a doping operation. The first resist 2804 is shown in a patternedcondition in FIG. 29 after the development step 3510. Dopant atoms orions 2902 are represented as vectors directed at the wafer 2802.

FIG. 30 is a plan view of the wafer shown in FIG. 29 showing dopedareas. The section plane corresponding to FIG. 29 is indicated on FIG.30.

In step 3512 the wafer 2802 is selectively doped to enhance theconductivity of selected areas. A first area 3004 that is doped will belocated at the top of a vibrating member 3304 (FIGS. 33, 34). Twoadditional areas 3002 that are doped will be used as electrodes to exertelectric forces on the vibrating member 3304 and capacitively couplesignals to and from the vibrating member 3304 (FIGS. 33, 34).

FIG. 31 is a sectional elevation view of the wafer 2802 shown in FIG. 29bearing a second resist 3102 that is being exposed to patterningradiation 2808.

In step 3514 the first resist 2804 is stripped from the wafer, and instep 3514 the second resist 3102 is applied to the wafer 2802. In step3520 the second resist is soft baked. In step 3520 the second resist isimagewise exposed to radiant or corpuscular energy 2808 using a secondmask 3104 in order to define areas for etching the wafer 2802.

In step 3522 the resist is developed. FIG. 32 is a sectional elevationview of the wafer shown in FIG. 31 after development of the secondresist. In FIG. 32 the second resist 3102 is seen in a patternedcondition.

In step 3524 the second resist 3102 is hard baked. The step of hardbaking makes the second resist 3102 more etch resistant so that overetching is reduced. In step 3526 the wafer 2802 is etched to form atrench 3302 (FIGS. 33, 34) proximate to a vibrating member 3304 (FIGS.33, 34). To fabricate resonators according to other embodiments of theinvention two or more trenches are etched rather than a single trench.

FIG. 33 is a sectional elevation view of the wafer shown in FIG. 32after etching using the second resist, and FIG. 34 is a plan view of aMEMS resonator device 3400 showing doped areas 3002, 3004 and an etchedrectangular trench 3302 surrounding a vibrating member 3304. The sectionplane used in FIG. 33 is indicated in FIG. 34. As seen in FIG. 34 asingle closed curve, rectangular plan trench 3302 surrounds thevibrating member 3004.

The length of the vibrating member 3004 is the vertical dimension of thevibrating member 3304 in the plan view shown in FIG. 34. The width ofthe vibrating member 3304 is the horizontal dimension of the vibratingmember 3304 in the plan view shown in FIG. 34. The height of thevibrating member 3304 is the vertical dimension of the vibrating member3304 in the sectional elevation view of shown in FIG. 33. The depth ofthe trench 3302 is equal to the height of the vibrating member 3304. Theratio of the height of the vibrating member 3304 to the width of thevibrating member is preferably at least about five more preferably atleast about ten. In order to achieve high ratios between the depth ofthe trench 3304 and its width, a reactive ion etcher (RIE) tool ispreferably used to form the trench 3304. Reactive ion etchers arecapable of etching trenches having aspect ratios of at least about fiftyto one. Using deep trench etching allows long beam to be fabricatedoriented perpendicularly to the wafer 2802 surface and occupy a smallarea of the wafer 2802. Additionally, by fabricating a vibrating memberthat is only attached to the wafer 2802 at its bottom, and making thevibration member long by using deep trench etching, a vibrating memberwith good Q can be obtained.

In operation the doped electrode areas 3002 can be used to connect theMEMS resonator 3400 to an external circuit such as an oscillator circuitin which the MEMS resonator serves as a frequency selective positivefeedback element. In an oscillator circuit, one of the electrodes 3002can be connected to the oscillators amplifier output, and the othercould be connected to the oscillators amplifier input to provide afeedback pathway. The resonance mode of the vibrating member 3304 is themode of a plate that is clamped at one end. The top of the vibratingmember 3304 (visible in FIG. 34) will oscillate back and forth along anaxis connecting the two doped electrode regions 3002. The vibratingmember 3304 exhibits resonances at one or more selected frequencies thatdepend on its dimensions, and the material properties of the siliconwafer from which it is made. The dimensions of the vibrating members inthis and other embodiments can be chosen to obtain a selected frequencyof vibration using principles of solid mechanics analysis. A FiniteElement Method (FEM) model based on solids mechanics principles can beused in selecting the dimensions of the vibrating member to obtain aselected frequency of vibration.

FIG. 36 is a fragmentary plan view of a MEMS 3600 resonator that hasvibratable plate 3602 oriented perpendicular to a semiconductor chipsurface 3626A. FIG. 37 is a sectional elevation view of the MEMSresonator 3600 shown in FIG. 36. The section plane of FIG. 37 isindicated in FIG. 36.

The MEMS resonator 3600 includes a vibrating plate 3602 that isdimensioned to support a vibration mode that includes five anti-nodes,and driven by five pairs of drive electrodes (including electrodes3606-3624) to vibrate in the vibration mode. The resonator 3600 comprisedeep trench 3604 etched in the surface 3626A of a semiconductor chip3626. The plan of the deep trench 3604 follows a closed curve path,specifically a rectangular path. The closed curve deep trench bounds thevibrating plate 3602. The vibrating plate 3602 includes a first freeside edge 3602A, a second free side edge 3602B, a free top edge 3602C(FIG. 37), and a bottom edge 3602D (FIG. 37) that is connected to thesemiconductor chip 3626. The vibrating plate 3602 further comprises afirst face 3602E and a second face 3602F. The vibrating plate 3602 isperpendicular to the surface 3626A of the semiconductor chip 3626. Inother words a vector normal to the first face 3602E is perpendicular toa vector normal to the surface 3626A of the semiconductor chip 3626. Thetop edge 3626C of vibrating plate 3602 is preferably selectively dopedto increase its conductivity and thereby enhance its electric forceinteraction with the drive electrodes 3606-3624. According to analternative embodiment of the invention the top edge 3626C is notselectively doped. In the latter case it is conductive due to the dopantpresent in the whole semiconductor chip 3626.

Five drive electrodes 3606-3614 are arranged from left to right in a rowon one side of the trench 3604 (top side in FIG. 36) opposite thevibrating plate 3602. Five more drive electrodes 3616-3624 are arrangedfrom left to right on an opposite side of the trench 3604 (bottom sidein FIG. 37) opposite the vibrating member 3602. The drive electrodes3606-3624 are preferably formed by selectively doping the semiconductorchip 3626. Pairs of electrodes that are directly across the vibratingplate 3602 from each other have opposite electrical phases. For examplethe first electrode 3606 in the top row 3606, and the first electrode3616 in the bottom row 3616 would differ in phase by Σ radians. Alsoeach electrode has an opposite electrical phase compared to electrodesthat are directly adjacent to it on the same side of the vibrating plate3602. Thus the electrical phase of the first electrode 3606 on the toprow 3606 would differ by Pi radians from the second electrode 3608 onthe top row 3608. In operation as the vibrating plate 3602 vibrates in amode with anti-nodes corresponding to the positions of the electrodes3606-3624, it will induce electrical signals in the electrodes 3606-3624with the relative phasing just mentioned. On the other hand ifelectrical signals with the relative phasing mentioned are applied tothe electrodes 3606-3624 the signals will induce the vibrating plate3602 to vibrate in the mode with anti-nodes corresponding the positionsof the electrodes. The doping of the top edge 3602C of the vibratingplate 3602C aids in the interaction with the electrodes 3606-3614. Asdiscussed above with reference to the resonators shown in FIGS. 13-16,the resonator shown in FIGS. 36-37 and other resonators discussed belowcan be coupled to external circuits in more than one alternative ways.With reference to the resonator 3600 shown in FIG. 36 one connectiontopology is to connect all the electrodes having one phase to one sideof an external circuit, and to connect all the other electrodes that areat opposite phase to the other side of the electrical circuit.

FIG. 38 is a fragmentary plan view of a third MEMS resonator 3800 whichhas a corrugated trench wall 3832. The resonator 3800 is fabricated inthe surface of a semiconductor chip 3834. The resonator 3800 includes avibrating plate 3802 that has a bottom edge 3802D connected to thesemiconductor chip 3834. The vibrating plate 3802 is rectangular inshape, and its three edges other than from the bottom edge are free. Thetop edge 3802 of the vibrating plate 3802 is doped to enhance itselectric field interaction with the electrodes 3806-3828. The corrugatedwall 3832 that is etched in the semiconductor chip 3834 surrounding thevibrating plate 3802, includes a number of inwardly projectingcorrugations 3830. A plurality of electrodes 3806-3828 that are made byselectively doping regions of the semiconductor chip 3834 are located onthe inwardly projected corrugations 3830 of the corrugated wall 3832.The inwardly projecting corrugations 3830 aid in electrically isolatingthe electrodes 3806-3828 from each other. The electrodes 3806-3828 arearranged in two rows of six. A first row includes six electrodes3806-3816 spaced along one side (top side in FIG. 38) of the resonator3800, facing the vibrating plate 3802. A second row includes six moreelectrodes 3818-3828 spaced along a second side (bottom side in FIG. 38)of the resonator 3800, facing the vibrating plate 3802. The electricalphase of every other electrode in each row is the same. Adjacentelectrodes within a row have phases that are Σ radians apart (i.e.,opposite phases). Electrodes that are across the vibrating plate 3802from each other also have opposite phases. The vibrating plate 3820vibrates in a mode that has six anti-nodes and five nodes. One node islocated at the longitudinal center 3802F of the vibrating plate 3820.

By virtue of the facts that the vibrating plate 3802 is symmetric asjudged from its longitudinal center 3802F, vibrates in a mode that has anode at its center, and vibrates in a mode that is anti-symmetric asjudged from its center, the effects of external jarring can be minimizedby connecting a first set of electrodes that has a first electricalphase to one input of a differential circuit (e.g. input of adifferential amplifier) and connecting a second set of electrodes thathas a phase opposite to that of the first set of electrodes to a secondinput of the differential circuit. When the resonator is jarred, thevibrating plate 3802 will tend to pick up a motion that is symmetric incontrast to its anti-symmetric vibration mode. The motion caused byjarring will cause common mode signals to be induced in the electrodesthat will be rejected by the differential circuit.

FIG. 39 is a fragmentary plan view of a MEMS resonator 3900 thatincludes a vibrating plate with two clamped edges. This MEMS resonator3900 has a open curve, specifically a U-shaped deep trench 3904 in asemiconductor chip 3926, partially (on three side) surrounding avibrating member 3902. The vibrating plate 3902 has two edges clamped,i.e., connected to the semiconductor chip 3922. A first side edge 3902A,and a bottom edge 3902D are connected to the semiconductor chip. A topedge 3902C and a second side edge 3902B are free. The vibrating plate3902 is dimensioned to vibrate in a mode that includes four anti-nodes.Four electrodes 3906-3912 are arranged in a first row along one side(top side in FIG. 39) of the trench 3904. Four more electrodes 3914-3920are arranged in a second row along a second side (Cower side in FIG. 39)of the trench 3904. One electrode from each of the rows is locatedadjacent to each of the four anti-nodes. The electrodes adjacent to eachanti-node, (one from each row) have opposite electrical phases. Within arow the electrical phase of the electrodes changes by half a cycle fromone electrode to the next. The top edge 3902C of the vibrating plate3902 is preferably doped so that the vibrating plate 3902 interacts withthe electrodes 3906-3920 via electric force interaction.

FIG. 40 is a fragmentary plan view of a MEMS resonator 4000 thatincludes a vibrating plate 4002 with three clamped edges. The resonator4000 is fabricated in a semiconductor chip 4022 and includes a vibratingplate 4002 bounded by a first deep trench 4004A on one side (top side inFIG. 40) and a second deep trench 4004B on a another side (bottom sidein FIG. 40). The vibrating plate 4002 is clamped (i.e., connected to thesilicon chip 4022) at a first side edge 4002A, a second side edge 4002B,and a bottom edge 4002D. The top edge 4002C is free. Four electrodes4006-4012 are arranged in a row on the semiconductor chip 4022 along theside of the first deep trench 4004A opposite the vibrating plate 4002.Four more electrodes 4014-4020 are arranged in a row on thesemiconductor chip 4022 along the side of the second deep trench 4004Bopposite the vibrating plate 4002. The eight electrodes 4006-4020 arepreferably formed by selectively doping the semiconductor chip 4022. Thetop edge 4002C of the plate 4002 is preferably also doped to enhance itselectric field interaction with the electrodes 4006-4020. The dopedregion of the top plate extends to a contact area 4024 beyond the sideedge 4002A of the plate 4002. The vibrating plate 4002 is dimensioned tovibrate in a mode that has four anti-nodes. One electrode from each rowis positioned at the longitudinal position of one of the anti-nodes. Thetwo electrodes positioned at each anti-node have opposite electricalphases. Within each row the electrical phase increases by half a cyclefrom one electrode in the row to the next.

FIG. 41 is a schematic of an oscillator 4100 using the MEMS resonator1600 shown in FIG. 16. An oscillator amplifier 4102 has an output 4102Acoupled to an output terminal 4104 and to an input 4106A of an impedancenetwork 4106. The impedance network 4106 can consist of a resistor. Theimpedance network serves to adjust the amplitude and optionally thephase of a fed back portion of the output of the oscillator amplifier4102. The impedance network has an output 4106B coupled to a first dopedconductive region 1630 of the resonator 1600, and to a first terminal4130A of a DC blocking capacitor 4130. A second terminal 4130B of the DCblocking capacitor 4130 is coupled to an input 4108A of a unity gaininverting amplifier 4108. An output 4108B of the unity gain invertingamplifier 4108 is coupled to a second doped conductive region 1628 ofthe resonator 1600.

Voltage dividers are used to bias the output of the signals output bythe impedance network 4106, and the unity gain inverting amplifier 4108.A first voltage divider that is used to bias the output of the impedancenetwork 4106 comprises a top resistor 4114 including a top terminal4114A coupled to a voltage source 4112, and a bottom terminal 4114Bcoupled to a first voltage divider midpoint 4116. A bottom resistor 4118includes a top terminal coupled to the first voltage divider midpoint4116, and a bottom terminal 4118B coupled through a first via 4120 tothe silicon base layer 202 (FIG. 16). The midpoint 4116 of the firstvoltage divider is coupled to the output 4106B of the impedance network4106.

A second voltage divider that is used to bias the output 4108B of theunity gain inverting amplifier 4108 comprises a top resistor 4122 thatincludes a top terminal 4122A coupled to the voltage source 4112, and asecond terminal 4122B coupled to a second voltage divider midpoint 4124.The second voltage divider further comprises a bottom resistor 4126 thatincludes a top terminal 4126A coupled to the second voltage dividermidpoint 4124, and a bottom terminal coupled through a second via 4128to silicon base layer 202 (FIG. 16). The second voltage divider midpointis coupled to the output 4108B of the inverting unity gain amplifier4108.

Biasing the first conductive region 1630, and the second conductiveregion 1628 using the first and second voltage dividers establishes anattractive force between the resonant member 1604, and the silicon baselayer 202 (FIG. 16). In operation of the oscillator, the attractiveforce will be modulated by periodic signals applied to the first andsecond conductive region 1630, 1628. The modulation of the attractiveforce, drives the flexural beam mode of the resonant member 1604.

The beam 1604 supports a transverse flexural vibration mode that has afirst anti-node over which the first doped region crosses and a secondanti-node over which the second doped region crosses. The first andsecond anti-nodes are located immediately adjacent (with no otherintervening anti-nodes) and on opposite sides of the center 1642 of thebeam 1604. The first and second anti-nodes have opposite phase, that is,they move in opposite directions (when one is deflected up the other isdeflected down). Connecting the first doped region 1630 with the outputof the impedance network 4106 directly, while driving the second dopedregion 1628 with the output Of the unity gain inverting amplifier 4106will drive the above mentioned flexural vibration mode.

A non-inverting input 4110A of a differential amplifier 4110 is coupledto a third doped region 1632 of the resonator 1600. An inverting input4110B of the differential amplifier 4110 is coupled to a fourth dopedregion 1626 of the resonator 1600. The third doped conductive 1632region crosses an anti-node located adjacent the first end 1604A of thebeam 1604. The fourth doped region 1626 crosses an anti-node adjacent tothe second end 1604B of the beam 1604. An output 4110C of thedifferential amplifier 4110 is coupled to an input 4102B of theoscillator amplifier 4102.

The impedance network 4106, unity gain inverting amplifier 4108,resonator 1600, and differential amplifier 4110 form a regenerativefeedback path for the oscillator amplifier 4102. A portion of an outputsignal of the amplifier oscillator 4102 is fed back through theregenerative feedback path to the input 4102B of the oscillatoramplifier 4102 causing it to output a periodic signal. In operation aperiodic signal i.e., the fed back signal, will pass through theresonator.

As discussed above differential signal connections to the resonator,such as shown in FIG. 41, are useful in lessening the effects of jarringmotion on the output of the oscillator 4100.

FIG. 42 is a schematic of an oscillator 4200 using the MEMS resonator4000 shown in FIG. 40. The circuit elements other than the resonator4000 are the same as shown in FIG. 41. Reference is made to thepreceding description of FIG. 41 for a description of those circuitelements. The coupling of the resonator 4000 of FIG. 40 to theoscillator circuit is as follows. The output 4106B of the impedancenetwork is coupled to a first electrode 4008, and a second electrode4018. The first electrode 4008 is located on the first side (top side inFIG. 42) of the resonator 4000 at the longitudinal position of a firstanti-node of the vibration mode of the vibrating plate 4002. The secondelectrode 4018 is located on the second side (bottom side in FIG. 42) ofthe resonator 4000 at the longitudinal position of a second anti-node.The first and second anti-nodes have opposite phase which is to say whenone is deflects up (from the perspective shown in FIG. 42) the otherdeflects down. The output 4108B of the unity gain inverting amplifier4108B is coupled to a third electrode 4010, and a fourth electrode 4016.The third electrode is located on the first side of the resonator 4000adjacent to the first electrode 4008, and across from the secondelectrode 4018, i.e., at the longitudinal position of the secondanti-node. The fourth electrode 4016 is located on the second side ofthe resonator 4000 adjacent to the second electrode 4018, and acrossfrom the first electrode 4008, i.e., at the longitudinal position of thefirst anti-node.

In the oscillator 4200, the bottom terminal 4118B of the bottom resistor4118 of the first voltage divider, the bottom terminal 4126B of thebottom resistor of the second voltage divider, and the extension of thedoped region 4024 are grounded to the semiconductor chip 4022.

In operation the first 4008 and second 4018 electrodes will receive thefed back signal at a first phase, and the third 4010, and fourth 4016electrodes will receive the fed back signal at a second phase thatdiffers from the first phase by one-hundred and eighty degrees.

The non-inverting input 4110A of the differential amplifier 4110 iscoupled to a fifth electrode 4014 and a sixth electrode 4012. The fifthelectrode 4014 is located on the second side of the resonator 4000adjacent to the fourth electrode 4016, on the side of the first sideedge 4002A of the vibrating plate 4002. The sixth electrode 4012 islocated on the first side of the resonator 4000 adjacent to the thirdelectrode 4010, on the side of the second side edge 4002B of thevibrating plate 4002. The inverting input 4110B of the differentialamplifier 4110 is coupled to a seventh electrode 4020, and an eighthelectrode 4006. The seventh electrode 4020 is located on the second sideof the resonator 4002 adjacent to the second electrode 4018 across fromthe sixth electrode 4012. The eighth electrode 4006 is located on thefirst side of the resonator 4002 adjacent to the first electrode 4008and across from the fifth electrode 4014.

The sixth 4012 and seventh 4020 electrodes correspond in position to athird anti-node of the vibration of the vibrating plate 4002 that hasthe same phase as the first anti-node. The fifth 4014 and eighth 4006electrodes correspond in position to a fourth anti-node of the vibrationof the vibrating plate that ha. The same phase as the second anti-node.

In operation the resonator 4000 serves as a frequency selective positivefeedback element in the feedback signal path of the oscillator 4200. Theresonator 4000 sets the frequency of the oscillator at the frequencycorresponding to the mode of vibration of the vibrating plate 4002 thathas anti-nodes positioned consistent with the positioning and phasing ofthe electrodes 4006-4002.

While the preferred and other embodiments of the invention have beenillustrated and described, it will be clear that the invention is not solimited. Numerous modifications, changes, variations, substitutions, andequivalents will occur to those of ordinary skill in the art withoutdeparting from the spirit and scope of the present invention as definedby the following claims.

What is claimed is:
 1. An electromechanical resonating devicecomprising: a first support member; a selectively doped vibrating memberthat is capable of resonating in a vibrational mode that has a firstnode and is attached to the first support at a position of the firstnode, the selectively doped vibrating member including; a first dopedconducting region extending from the first support; and an insulatingregion; the selectively doped vibrating member comprising a body ofsingle crystal semiconductor material; the support comprising a toplayer of single crystal semiconductor material that is contiguous withthe body of single crystal semiconductor material; and the support beingcontiguous with a portion of single crystal silicon that overlies alower layer of silicon di-oxide.
 2. The electromechanical resonatingdevice according to claim 1, whereins: the selectively doped vibratingmember comprises a beam shaped member having a first end, and a secondend, a first longitudinal side extending between the first end and thesecond end, and a second longitudinal side extending between the firstend and the second end.
 3. The electromechanical resonating deviceaccording to claim 2 wherein: the selectively doped vibrating member iscapable of resonating in a vibrational mode that includes the first nodeand a second node; and the resonating device further comprises a secondsupport attached at the second node.
 4. The electromechanical resonatingdevice according to claim 3 wherein: the first doped conducting regionextends from the first support to the second support.
 5. Theelectromechanical resonating device according to claim 3 wherein: thefirst support is attached to the first longitudinal side; the secondsupport is attached to the second longitudinal side; and the devicefurther comprises: a third support attached to the second longitudinalside at the first node; and a fourth support attached to the firstlongitudinal side at the second node.
 6. The electromechanicalresonating device according to claim 2 wherein: the first node islocated at approximately a center of the beam shaped member; the firstsupport is attached at approximately a center of the first longitudinalside of the beam shaped member; and the first doped conducting regionextends from the first support towards the first end of the beam shapedmember; and the resonating device further comprises: a second supportattached at approximately the center of the second longitudinal side ofthe beam shaped member; and a second doped conducting region extendingfrom the second support toward the second end of the beam shaped member;and an insulating region between the first doped conducting region andthe second doped conducting region.
 7. The electromechanical resonatingdevice according to claim 6 wherein: the selectively doped vibratingmember is capable of resonating in a vibrational mode that includes thefirst node, a second node, and a third node; and the resonating devicefurther comprises; a third support attached to the beam at the secondnode; a fourth support attached to the beam at the third node.
 8. Theelectromechanical resonating device according to claim 7 wherein: thefirst doped conducting region extends from the first support to thethird support; and the second doped conducting region extends from thesecond support to the fourth support.
 9. An electromechanical resonatingsystem comprising: a vibrating member that is capable of resonating in avibrational mode that includes a first anti-node characterized by afirst relative phase and a second anti-node characterized by a secondrelative phase that is opposite to the first phase; a first electrodepositioned in a vicinity of the first anti-node; a second electrodepositioned in a vicinity of the second anti-node; and an electriccircuit including: a differential amplifier having: a first differentialinput coupled to the first electrode; and a second differential inputcoupled to the second electrode.
 10. The electromechanical resonatingsystem according to claim 9 wherein: the first electrode comprises: afirst selectively doped region of the vibrating member.
 11. Theelectromechanical resonating system according to claim 10 wherein: thesecond electrode comprises: a second selectively doped region of thevibrating member.
 12. The electromechanical resonating system accordingto claim 9 wherein: the vibrating member comprises: a beam capable ofvibrating in a flexural mode having at least two anti-nodes, and one ormore nodes.
 13. The electromechanical resonating system according toclaim 12 wherein: a longitudinal coordinate of a center of gravity ofthe beam corresponds to a position of one of the one or more nodes. 14.The electromechanical resonating system according to claim 12 furthercomprising: one or more supports coupled to the beam at each of the oneor more nodes.
 15. An electromechanical resonating system comprising: avibrating member that is capable of resonating in a vibrational modethat includes a first anti-node characterized by a first relative phaseand a second anti-node characterized by a second relative phase that isopposite to the first phase; a first electrode positioned in a vicinityof the first anti-node; a second electrode positioned in a vicinity ofthe second anti-node; and the vibrating member comprises a beam capableof vibrating in a flexural mode having an even number of anti-nodesincluding a first plurality of anti-nodes characterized by the firstphase, and a second plurality of anti-nodes characterized by the secondphase; a first set of electrodes each positioned in a vicinity of one ofa set of the first plurality of anti-nodes; and an electric circuitincluding: a first input coupled to the first set of electrodes.
 16. Theelectromechanical resonating system according to claim 15 furthercomprising: a second set of electrodes each positioned in the vicinityof one of a set of the second plurality of anti-nodes; and a secondinput of the electric circuit coupled to the second set of electrodes.17. An electromechanical resonating device comprising: a first supportmember; a selectively doped vibrating member that is capable ofresonating in a vibrational mode that has a first node and is attachedto the first support at a position of the first node, the selectivelydoped vibrating member including; a beam shaped member having a firstend, and a second end, a first longitudinal side extending between thefirst end and the second end, and a second longitudinal side extendingbetween the first end and the second end; a first doped conductingregion extending from the first support; and an insulating region; andwherein the first node is located at approximately a center of the beamshaped member; the first support is attached at approximately a centerof the first longitudinal side of the beam shaped member; and the firstdoped conducting region extends from the first support towards the firstend of the beam shaped member; and the resonating device furthercomprises: a second support attached at approximately the center of thesecond longitudinal side of the beam shaped member; and a second dopedconducting region extending from the second support toward the secondend of the beam shaped member; and an insulating region between thefirst doped conducting region and the second doped conducting region.18. The electromechanical resonating device according to claim 17wherein: the selectively doped vibrating member is capable of resonatingin a vibrational mode that includes the first node, a second node, and athird node; and the resonating device further comprises; a third supportattached to the beam at the second node; a fourth support attached tothe beam at the third node.